1. Field of the Invention
The present invention relates to an improvement in an outer rotor type disc driving spindle motor on which a plurality of discs such as magnetic discs are to be laminated and mounted.
2. Description of the Related Art
A rotor diameter of an outer rotor type disc driving spindle motor is increased due to its structure. An inertia moment is increased so that a rotational non-uniformity is reduced. Accordingly, the outer rotor type disc driving spindle motor is suitable as a magnetic disc driving motor to be used, for instance, as a memory medium for a computer.
There has been provided conventionally a disc driving spindle motor on which a plurality (about ten) of magnetic discs are able to be loaded in lamination in a magnetic disc driving device called xe2x80x9c1.6 Inch-Heightxe2x80x9d with 40.6 mm in total height.
Such a conventional spindle motor for driving a disc is shown in FIG. 4.
As shown in FIG. 4, in an outer rotor type disc driving spindle motor, a rotor 3 (hub) having magnets 3a that face a stator 2 having coils 2a (stator coils) are located outside the stator 2 fixedly supported to a shaft. The energization of the stator 2 to cause rotation of rotor 3 (hub) by interaction of the stator 2 with magnets 3a is caused by controlled electrical signals provided on wires to coil windings.
In such a conventional spindle motor for driving a disc, the rotor 3 and the stator 2 are provided with the following support structure.
The rotor 3 is formed to have a substantially cylindrical drum portion 3b in which an axial dimension is set so that a plurality (predetermined number, e.g., ten in this case) of discs (not shown) may be laminated in the axial direction and loaded on its outer circumference. Then, the rotor 3 is rotatably supported on the shaft 1 through an upper bearing 4 located on an opening surface 3c on the upper side and a lower bearing 5 located at an opening surface 3d on the lower side of the drum portion 3b of FIG. 4.
In this case, a substantially annular bush 6 is interposed between opening ends of the lower opening surface 3d and the lower bearing 5. Accordingly, the lower end portion of the rotor 3 is supported by the lower bearing 5 through the bush 6. The hub is indirectly supported by the bearing 5 via a bush 6.
The stator 2 is disposed between the upper and lower bearings 4 and 5. In this stator 2, the axial dimension S2 is set to be substantially the same as the axial dimension M2 of the magnets 3a.The stator 2 is fixedly supported on the shaft 1 over the range of the full axial dimension S2.
Incidentally, reference character 1a denotes a tubular hole formed in the shaft 1 and 2a1 denotes a lead wire connected to a coil end of the stator coils 2a.This lead wire 2a1 is inserted into the tubular hole 1a to be guided to the outside of the rotor 3 (drum portion 3b). Reference numeral 7 denotes a frame for holding and fixing the shaft 1, and the frame 7 is located so as to cover the bush 6.
Any one of the bearings 4 and 5 is a radial bearing as shown in FIG. 4.
The magnet 3a is made of magnet material having a magnetic energy product of 160 kJ/m3 that is a typical example of this type motor. The magnet 3a is fixed to an inner wall of the drum portion 3b of the rotor 3 with an axial dimension that is close to the full length between the upper and lower bearings 4 and 5.
However, the prior art suffers from the following disadvantages.
Since the stator 2 is interposed between the upper and lower bearings 4 and 5, the lead wire 2a1 from the stator coils 2a has to be led to the outside of the rotor 3 through the tubular hole la of the shaft 1. For this reason, it is necessary to perform the processing of the shaft 1, the insertion of the lead wire 2a1 through the tubular hole 1a, and the like, thereby increasing both the part cost and the assembling work cost. Also, the structure itself in which the stator 2 is interposed between the upper and lower bearings 4 and 5 causes labors for assembling the stator 2 to be increased.
Since the bush 6 is interposed between the lower bearing 5 and the rotor 3, the assembling precision and hence the rotational precision would be degraded. It gives trouble to enhance the precision. In addition, since the bush 6 is an expensive and precise part, the part cost and the assembling work cost are both increased.
In particular, such a complicated structure, in which the annular hole 1a is formed in the shaft 1, the lead wire 2a1 is passed through the annular hole 1a, and the bush 6 is provided in addition to the frame 7, results in more difficulty and causes a cost increase in the assembly.
Incidentally, it is possible to solve the above mentioned problem by disposing the stator 2 under the lower bearings 4 and 5 by providing the stator 2 outside (on the side of the opening surface 3d on the lower side of the drum portion) rather than between the upper and lower bearings 4 and 5. This will now be explained.
In the motor shown in FIG. 4, assume that the relationships, L:S2=1:0.38 and L:B2=1:0.66, are established where L is the length of the shaft 1 (nearly equal to the total motor length) and B2 is the distance between the bearings 4 and 5.
Keeping the shaft length L intact, i.e., under the condition that the motor is not enlarged, the value S2 to the value L (the dimension in the axial direction of the stator) and the distance between the bearings 4 and 5 (referred to as B3 for the sake of explanation although not shown) are sought in the case where the stator 2 is shifted from the distance between the bearings 4 and 5 to the outside. Then, the relationship, L:S2=1:0.38, is unchanged but the relationship, L:B3=1:0.22, is established.
Since the axial dimension S2 of the stator is too large, the ratio of the dimension B3 between the bearings 4 and 5 to the shaft length (nearly equal to the total motor length) is extremely small at 0.22. For this reason, the center between the bearing 4 and 5 is remarkably eccentric from the center of gravity of the rotor 3 on which the discs are mounted, as a result of which the bearing load is displaced to adversely affect the rotational precision or the service life of the bearings.
It is basically preferable that the discs be located in a place corresponding to a position between the bearings 4 and 5. If the distance (ratio) between the bearings 4 and 5 is reduced, the number of the discs that may be mounted is also reduced. Accordingly, it is preferable that the ratio L:B of the distance B between the bearings 4 and 5 to the dimension L be large, and in general, the ratio is needed to be about 0.5 at the minimum.
However, as described above, such a simple method to shift the stator 2 from the space between the bearings 4 and 5 to the outside is not actually adopted because the ratio is extremely small at 0.22. Accordingly, the above-described problems could not yet solved in the state of the art.
In order to solve the problems inherent in the above-described prior art, an object of the present invention is to provide a spindle motor for driving discs, which may reduce the part cost and the assembling work cost and enhance the assembling precision and the rotational precision.
In order to attain this and other objects, according to a first aspect of the present invention, there is provided an outer rotor type disc driving spindle motor in which a rotor having a magnet facing a stator having a coil and supported to a shaft is located outside of the stator, characterized in that: the rotor is formed to have a cylindrical drum portion to have an axial dimension so that a predetermined number of discs may be laminated in the axial direction and mounted around an outer circumference of the rotor, the rotor being rotatably mounted on the shaft through a first bearing positioned in the vicinity of one of opening surfaces of the drum portion and a second bearing positioned in a predetermined position on a side of the other opening surface, the stator is disposed on the side of the other opening surface beyond the second bearing and is fixed to a frame for holding the shaft in the vicinity of the other opening surface to be supported by the shaft through the frame, an end surface position on the side of the second bearing is set to be at the same position as a position of the laminate surface position of the discs set in a predetermined position, on the side of the other opening surface, of the outer circumference of the drum portion or closer to the side of the other opening surface, with an outer diameter of the stator being set to be smaller than an outer diameter of the drum portion, the magnet has a magnetic energy product such that a ratio of a distance between the first and second bearings to a length of the shaft (nearly equal to the total motor length) maybe set at 0.5 or more, and a spacer is disposed between outer races of the first and second bearings.
According to a second aspect of the invention, in the first aspect of the invention, it is characterized that the spacer is a flanged portion formed to project from an inner wall of the drum portion of the rotor.
According to a third aspect of the invention, in the first aspect of the invention, it is characterized that each of the outer races of the first and second bearings is formed integrally with the spacer and an inner race of the second bearing is formed integrally with the shaft.
According to the first aspect of the invention, the stator is disposed on the opposite side to the space between the first and second bearings within the rotor drum portion, i.e., on the side of the other opening surface of the drum portion.
Accordingly, it is possible to lead the coil end of the stator coil to the outside of the drum portion without passing through the bearings. Thus, it is possible to dispense with the processing of the shaft, the work for passing the lead wire through the tubular hole and the like, thereby reducing both the part cost and the assembling work cost.
With such an arrangement, it is possible to facilitate the assembling work of the stator in comparison with the prior art in which the stator is disposed between the first and second bearings. According to the present invention, the stator is fixed to the frame for holding the shaft to be supported by the shaft through the frame. If the stator is fixed to the frame in advance, the stator is supported by the shaft by coupling the frame and the shaft. Accordingly, the assembling work of the stator may be further facilitated.
Any bush is not provided between the opening ends of the opening surfaces of the drum portion or between the bearings, and the drum portion is supported by the shaft through the first and second bearings. Accordingly, it is possible to avoid the degradation not only in assembling precision but also in rotational precision and further, to reduce both the part cost and the assembling work cost. Incidentally, the spacer functions to position and fix the first and second bearings.
If the end position on the second bearing side of the stator is set on the side of one of the opening surfaces beyond the laminate surface position of the discs, the second bearing has to be set at the position displaced toward the first bearing correspondingly to reduce the interval between the first and second bearings.
According to the first aspect, in contrast thereto, the end surface position of the stator on the second bearing side is set at the same position as the laminate surface position of the discs or closer to the other opening surface from the laminate surface position, it is possible to enlarge the distance between the first and second bearings.
With such an arrangement, it is possible to ensure the stable rotation of the rotor drum portion to mount a larger number of the discs thereon. Inversely, if the number of the discs to be mounted is determined, it is possible to shorten the axial dimension of the rotor drum portion (shaft length) to mount the same number of the discs and to miniaturize the motor as a whole in the axial direction.
The larger the magnetic energy product of the magnet facing the stator is, the smaller the axial dimension S of the stator will become. The ratio L:B of the distance between the first and second bearings to the shaft length L (nearly equal to the total motor length) can be set to be large.
As a result, it is possible to increase the distance between the bearings and to rotate the rotor in a stable manner to enhance the rigidity and rotational precision.
According to the first aspect of the invention, the magnet has the magnetic energy product that exceeds the conventional value of 160 kJ/m3. In the case where the above-described ratio L:B is obtained while keeping the other conditions such as the magnetic characteristics of the stator or the like unchanged, it is possible to realize the value 0.5 or more that is generally needed.
With such a structure, it is possible to facilitate the arrangement of the stator to the other opening surface side. In addition, it is possible to further shorten the axial dimension of the rotor drum portion that is needed to mount the constant number of the discs with the increase in the distance between the first and second bearings.
According to the first aspect of the invention, it is possible to shorten both the axial dimension and the dimension perpendicular to the axial direction of the motor as a whole. Namely, it is possible to miniaturize the overall contour of the motor. At this time, it is unnecessary to reduce the number of the discs to be mountable and also, there is no fear of the unstable rotation about the axis of the rotor.
According to the second aspect of the invention, the flanged portion is formed to project from the inner wall of the drum portion of the rotor, and this is used as the space according to the first aspect. It is thus possible to reduce the number of the mechanical parts to simplify the structure.
According to the third aspect of the invention, the first and second bearings, the spacer and the shaft are formed integrally. It is thus possible to further reduce the number of the mechanical parts and to simplify the structure to enhance the rotational precision. Furthermore, the motor precision is free from the adverse affect of the change in environmental temperature.